Growth Hormone Physiology Across the Lifespan

Growth hormone (GH) is released from somatotroph cells in the anterior pituitary. Growth-hormone-releasing hormone (GHRH) stimulates release, while somatostatin inhibits it. GH secretion occurs in pulses rather than as a steady infusion, with larger pulses often occurring during sleep. That pulsatile pattern explains an important diagnostic point: a random GH level can be very low in a healthy person simply because the blood sample was drawn between pulses. Random GH testing does not diagnose GH deficiency. GH acts directly on several tissues, but much of its growth-promoting effect occurs through insulin-like growth factor 1 (IGF-1). GH signals the liver, and other peripheral tissues, to produce IGF-1. IGF-1 then promotes cartilage growth at the epiphyseal plates, protein synthesis, and growth of bone and soft tissue. GH also has metabolic effects that can oppose insulin. It promotes lipolysis and can reduce insulin sensitivity, which helps explain why somatropin can raise blood glucose or worsen diabetes. IGF-1 provides feedback to the hypothalamus and pituitary, helping limit further GH release. A useful way to picture the axis is that GH sends the growth signal,...